Fraction of myosin heads bound to thin filaments in rigor fibrils from insect flight and vertebrate muscles

The binding of myosin heads to actin in rigor striated muscle is affected by steric constraints imposed by the structure of the filament lattice and the mismatch of the helical periodicities of the thick and thin filaments1,2. In rabbit fibres, despite these steric constraints, at least 95% of the myosin heads are attached to actin3–5. It has been suggested1,2,6 that not all the myosin heads in insect flight muscle may be able to bind to the thin filament in rigor conditions. Here we compare the fraction of heads bound in the rigor state in the flight muscle from the blowfly (Sarcophaga bullata) and in striated muscle from the frogs Rana pipiens and Rana temporaria, using a tryptic digestion technique4,78. We find that whereas at least 95% of the heads are bound tightly in the frog muscle, only 70% of the heads are bound in rigor Sarcophaga muscle.